The present invention relates to periodic-permanent-magnet (PPM) focused traveling wave tubes (TWTs), and more particularly to improvements to the magnetic circuits of such devices.
A conventional PPM TWT comprises a plurality of pole pieces, which have the dual functions of providing the magnetic field for focusing the electron beam and forming parts of the tuned r.f. cavities of the TWT. In a typical example, the magnetic field is produced by a periodic series of sets of four cylindrical Samarium-Cobalt magnets which are arranged in a pattern about the beam axis with four-fold symmetry. The symmetrical arrangement of the magnets in a set is disturbed in cavities where the r.f. energy is fed in or removed; these are known as the coupler and sever cavities. The r.f. energy is typically fed in or removed from the coupler cavities through reduced height waveguide sections. The sever cavities are typically filled with a lossy ceramic material having a width about equal to the width of the waveguide sections, and serve to attenuate the r.f. signal to enhance the stability of the TWT operation.
In a conventional PPM TWT, to accommodate the waveguide elements at the coupler cavities and the lossy ceramic material at the sever cavities, the Samarium-Cobalt magnets are displaced asymmetrically from the nominal positions having four-fold symmetry and protrude to varying degrees over the outward edges of the pole pieces. This magnet rearrangement reduces the magnetic field on the tube axis and introduces undesirable transverse magnetic field components which deflect some of the electrons into the sides of the circuit. Circuit heating is increased as a result of the beam interception, and the power handling margin of the tube is decreased. Further, the power output of the tube is decreased since the electrons which are intercepted no longer interact with the r.f. circuit.
Currently, the performance of virtually every PPM TWT needs to be improved by shunting. This procedure involves the placement of small rectangular strips of iron at experimentally determined locations along the magnetic circuit to compensate for the departure from rotational symmetry produced by the magnetic misalignment at the coupler and sever cavities and by other defects. The need for shunting arises primarily from transverse mechanical misalignments and magnetic inhomogenieties.
It would therefore represent an advance in the art to provide a magnetic circuit for a PPM TWT which has its rotational symmetry only minimally disturbed by the couplers and severs.
It would further be advantageous to provide a magnetic circuit for a PPM TWT which allows the electron beam to remain centered on the tube axis, minimizes beam interception and the amount of shunting required to align the beam on the tube axis, reduces circuit heating and increases the power handling margin of the TWT.